Today, many electronic product assemblies are manufactured from components or sub-assemblies provided to a manufacturer by a vendor/subcontractor. A common method of assembly of such products is to connect leads/terminals of the components or subsystem printed wiring assemblies (PWAs) onto the main system PWB by mass soldering. The two most common continuous mass soldering processes are wave soldering and reflow soldering. Wave soldering is commonly used when a high mix of through-hole components are involved in the product assemblies. On the other hand, the reflow soldering process is employed when most components are surface-mounted. In reflow soldering, the entire product assembly is subjected to a specific temperature profile sufficient to reliably melt the solder, forming the interconnects between the main system PWB and the subsystem PWB.
The most common standard tin/lead (60/40 or 63/37 Sn/Pb) solder is an alloy the melting temperature (183.degree. C.) of which is lower than either of its pure components' melting points. That is, Sn melts at 232.degree. C. and Pb melts at 328.degree. C. A wide range of solder alloy compositions, ranging from 80 percent Sn with 20 percent Pb to 15 percent Sn with 85 percent Pb, has the solidus melting point of 183.degree. C. When the encapsulated subsystems or components are subjected to a standard 60/40 or 63/37 Sn/Pb temperature profile, their internal components can reach a temperature of about 20.degree. C. above the melting temperature of the solder alloy. Obviously, if the same solder alloy is used in the manufacture of a subsystem printed wiring assembly, then the solder joints of the components in the subsystem will melt at the same time when the new joints between the main product assembly and the subsystem PWB are formed. Such a condition can cause solder joints on the encapsulated subsystem PWB to fail or components to move when the solder is in the liquidus state. Therefore, with most subassemblies the encapsulated subcontractor-supplied PWAs must provide a thermal guard band between the internal solder joint temperature of the subsystem PWAs and the melting temperature of the solder alloy used in the assembly of the subsystem, especially when the subsystem is subjected to a standard 60/40 Sn/Pb reflow process. This guard band is 20.degree. C. minimum.
One approach to a higher melting point for soldering subsystem boards is to move toward a lead-rich solder, i.e., Pb &gt;85 percent. However, at a 90 percent lead alloy, lead/tin solder melts at 268.degree. C. Unfortunately, this temperature is in excess of the temperature to which most electronic components are certified. Therefore, one is led to tin-rich solder alloys. These alloys typically melt at temperatures below 250.degree. C. However, virtually all components received from suppliers have metalization on the connecting leads that contains some lead.
Referring now to FIG. 1, illustrated is a conventional solder joint of a surface-mounted component on a PWB. When a component lead 110 is conventionally soldered to a copper trace 130 on a PWB 140, lead in the presence of tin combines with other impurities to form tertiary alloys at an intermetallic layer 120 between a solder ball 150 and the copper trace 130. The tertiary alloys may be of tin/lead in combination with silver, bismuth, antimony, or indium. These tertiary alloys have significantly lower melting points than the eutectic point of the reflow tin/lead solder, i.e., 183.degree. C. Thus, when reflow is accomplished, these tertiary alloys melt before the process temperature required to assure reflow is achieved, and the subsystem joints fail prematurely. This can cause a component to move from the desired location on the PWB.
Accordingly, what is needed in the art is a low cost method for forming reliable, lead-free solder joints on encapsulated subsystem PWBs that will be subjected to a reflow process.